7-Amino-5-fluoro-3-Beta-D-ribofuranosylpyrazolo[4,3-d]pyrimidine (1), the corresponding 7-amino-5-chloro compound (2), and several other previously unknown analogs of the C-nucleoside antibiotic formycin will be prepared via semi-synthetic modification of the natural product or by total synthesis, and will be evaluated as cytotoxic and antineo-plastic agents in vitro and in vivo. In addition to being resistant to purine nucleoside phosphorylase (PNP) by virtue of their C-nucleoside structure, 1 and 2 are expected to resemble 2-fluoroadenine nucleosides in not being adenosine deaminase (ADA) substrates while retaining the ability to be 5'-phosphorylated by adenosine kinase (AK). The 5'-triphosphates of 1 and 2 may act like formycin 5'triphosphate in blocking de novo purine biosynthesis via decreased PRPP production from ribose-5-phosphate and ATP, or in blocking purine salvage via feedback inhibition of HGPRTase. Incorporation into RNA may also occur, giving modified RNA species and impairing portein synthesis; alternatively, the non-phosphorylated C-nucleosides may act via suicide inactivation of S-adenosyl L-homocysteine (SAH) hydrolase. The 2'-deoxy compounds (3,4) derived from 1 and 2 are likewise expected to be PNP and ADA resistant, while being potential substrates for 2'-deoxycytidine kinase (dCK). By analogy with dATP, the 5'triphosphates of 3 and 4 may cause dCTP depletion via ribonucleotide reductase feedback inhibition, or else may compete with dATP as DNA polymerase substrates. 2'-Deoxyribo-C-nucleoside analogs (5,6) of deosyguanosine and deoxy-6-thioguanosine will also be synthesized, since these are of particular interest as potential agents against human T-cell leukemias. Lack of ADA in T-lymphocytes of patients with genetic immunodeficiency disease is known to lead to accumulation of toxic dATP levels; similarly, absence of PNP results in toxic concentrations of dGTP. Thus it has been proposed that ADA-stable and PNP-stable analogs of dATP and dGTP may cause the same toxic effects in malignant cells with normal ADA and PNP levels as are seen in T-lymphocytes of genetically immunodeficient patients. A number of T-cell mutants lacking one or more of the enzymes that activate or deactivate purine nucleosides are available and will be used to identify the biochemical site(s) of action of the analogs. These studies are intended to increase the understanding of the role of natural purine deoxyribonucleosides in T-cell function, and to lead to new agents of potential benefit in human cancer medicine.